U.S. Pat. No. 3,916,421, included herein by reference thereto, describes an ink jet recording device in which an ink jet issues under high pressure from a nozzle and breaks up into a train of drops at a point of drop formation inside a control electrode. This train of normally uncharged drops travels in a line or along an initial axis towards an ink receiving surface, as a recording medium, e.g. a sheet of paper, which is mounted on or otherwise affixed to a support movable relative to the nozzle, e.g. a rotating drum of a drum plotter. On the way from the nozzle to the ink receiving surface, the drops pass a transverse electric field generated between a negatively charged high voltage electrode and a lower part of the control electrode. Now, if a positive control voltage is applied to the control electrode while the ink in the nozzle is grounded, an electric field is established at the point of drop formation causing each of the drops formed at the point of drop formation to be negatively charged. Because of the charge, these drops are deflected into a catcher or gutter and cannot reach the ink receiving surface. Thus, the length of time during which the signal voltage or "print pulse" applied to the control electrode is zero or less than a cut-off control voltage, determines the number of drops that reach an elementary area (pixel area) of the receiving surface, which is aligned with the ink jet axis. Thus, the printing pulses control the amount of the ink laid down on the individual pixel areas and therefore the densities of the pixels which in turn may form a halftone image. An improvement of the ink jet apparatus mentioned above is described in U.S. Pat. No. 4,620,196 also included herein by reference thereto. In this improved ink jet apparatus, the rate and position of the drop formation is controlled by ultrasonic stimulation of the ink jet. Further, the length of the electric print pulses determining the number of drops that reach the receiving surface is adjusted such that it equals n/f, where f is the drop formation rate which is equal to the ultrasonic stimulation frequency (e.g. 1 MHz) and n is an integer chosen such that the ratio n/f is close to the length of the original print signal. Additionally the start of the print pulse is synchronized with a suitable phase of the ultrasonic stimulation. This ensures the start of the print pulse always coinsides with the same phase of the drop formation process. The effect of these measures is an appreciable reduction of the draininess of the half tone image formed by the printed pixels.
It has further been proposed to synchronize the drop formation rate and, thus, the printing pulses, with the pixel rate which is controlled in dependence of the relative movement between the nozzle and the ink receiving surface, i.e. in the case of a drum plotter by means of a shaft encoder. This reduces the draininess of the printed image.
The electrical charge which an individual droplet receives when a given potential difference is applied between the ink jet and the control electrode depends to a great extent on the relationship between the time of formation of the droplet under consideration and the time of application of the potential difference. In the case of a stimulated jet, where the drop formation rate is controlled by an ultrasonic stimulation signal of predetermined frequency, the amount of electrical charge which is applied to the first droplet separated from the continuous portion of the jet after the occurrence of the leading edge of a printing pulse is ultimately a function of the phase angle of the stimulation signal period at which the leading edge of the print pulse occurs.
U.S. Pat. No. 4,620,196 mentioned above discloses means for synchronizing the start of the print pulse with a suitable phase of the ultrasonic stimulation. This synchronization must be adjusted by highly trained personal. Further, the synchronization established in the factory or at the beginning of a recording process to yield optimum results may become insatisfactory when parameters, such as the temperature, pressure, viscosity and composition of the ink change during the recording process. Thus, it is desirable to provide a method and an apparatus by which the relationship between the drop formation and the occurrence of the leading edge of the print pulses can be adjusted in short intervals to provide for the application of a desired amount of charge to the drop formed after the occurrence of the leading edge of a print pulse.